When manufacturing custom CNC machining parts, holes with screw threads are often required for fastening the finished part or facilitating the building. One of the most common ways to produce the thread holes is to drill a hole of the right size with a drill bit and tap it with a tap. People would need a drill size chart and tap size chart to determine the proper tap drill for a certain thread size or proper tap for an existing hole. The drill size chart is a table list standard size drill bits in several measurement systems, including fractional, metric, wire gauge number, and letter. Fractional sizes are measured in inches, while metric sizes are measured in millimeters. The wire gauge and letter systems refer to tool diameters that increase as the wire gauge decreases from #107 to #1 and then continues from A to Z. The decimal equivalents of the diameters are shown in both English and Metric units. The “M” indicates the “metric diameter” of its thread (see tap drill chart below), which in our case is 4 mm. The second number (0.7) is the pitch which corresponds to the distance between two adjacent threads in millimeters. Finally, 20 is the length of the bolt in mm, measured from the tip of the thread to the bottom of the head (with the exception of oval headed bolts). Unfortunately, due to the shape it can’t create a thread all the way to the end of the hole. So next comes the bottoming tap that has the capacity to do this. However, the bottoming tap cannot start the threading on an uncut hole, so it can only take part in the cutting process as the second or third tap. The taper tap is only used in certain situations such as when the material is very hard or the hole is too small. The taper tap features a smooth diameter transition that allows for a less aggressive cutting action. This minimises the risk of damaging the drilled hole. If the taper needs to go first, the intermediate tap follows second and the thread is finished with the bottoming tap. Thread Geometry And Drilling The Hole Some prefer to use somewhat arbitrary but reliable formulas like: “Dtap = Nominal Diameter – Thread Pitch”. This formula works for both metric and imperial units, and for all 60-degree threads (all ISO, DIN, UTS, UNC, UNS, and UNF bolts). The first tap drill chart below is in mm and the next in inches. Both are available as a printable tap drill chart PDF as well. Tap Drill Chart (mm and inches)

As per the ISO metric screw thread standard, the thread depth should be 0.614 x pitch, which is why a smaller pitch number also results in a lower thread depth value. Now engineers can sit down and manually calculate the right tap drill. Or they can consult a tap drill chart like those found in the next section.